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<a href="intro.html"><h2>SQLite C Interface</h2></a><h2>Mutexes</h2><blockquote><pre>sqlite3_mutex *sqlite3_mutex_alloc(int);
void sqlite3_mutex_free(sqlite3_mutex*);
void sqlite3_mutex_enter(sqlite3_mutex*);
int sqlite3_mutex_try(sqlite3_mutex*);
void sqlite3_mutex_leave(sqlite3_mutex*);
</pre></blockquote><p>
The SQLite core uses these routines for thread
synchronization. Though they are intended for internal
use by SQLite, code that links against SQLite is
permitted to use any of these routines.</p>

<p>The SQLite source code contains multiple implementations
of these mutex routines.  An appropriate implementation
is selected automatically at compile-time.  The following
implementations are available in the SQLite core:</p>

<p><ul>
<li>   SQLITE_MUTEX_PTHREADS
<li>   SQLITE_MUTEX_W32
<li>   SQLITE_MUTEX_NOOP
</ul></p>

<p>The SQLITE_MUTEX_NOOP implementation is a set of routines
that does no real locking and is appropriate for use in
a single-threaded application.  The SQLITE_MUTEX_PTHREADS and
SQLITE_MUTEX_W32 implementations are appropriate for use on Unix
and Windows.</p>

<p>If SQLite is compiled with the SQLITE_MUTEX_APPDEF preprocessor
macro defined (with "-DSQLITE_MUTEX_APPDEF=1"), then no mutex
implementation is included with the library. In this case the
application must supply a custom mutex implementation using the
<a href="../c3ref/c_config_getmalloc.html#sqliteconfigmutex">SQLITE_CONFIG_MUTEX</a> option of the sqlite3_config() function
before calling sqlite3_initialize() or any other public sqlite3_
function that calls sqlite3_initialize().</p>

<p>The sqlite3_mutex_alloc() routine allocates a new
mutex and returns a pointer to it. If it returns NULL
that means that a mutex could not be allocated.  SQLite
will unwind its stack and return an error.  The argument
to sqlite3_mutex_alloc() is one of these integer constants:</p>

<p><ul>
<li>  SQLITE_MUTEX_FAST
<li>  SQLITE_MUTEX_RECURSIVE
<li>  SQLITE_MUTEX_STATIC_MASTER
<li>  SQLITE_MUTEX_STATIC_MEM
<li>  SQLITE_MUTEX_STATIC_MEM2
<li>  SQLITE_MUTEX_STATIC_PRNG
<li>  SQLITE_MUTEX_STATIC_LRU
<li>  SQLITE_MUTEX_STATIC_LRU2
</ul></p>

<p>The first two constants (SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE)
cause sqlite3_mutex_alloc() to create
a new mutex.  The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
is used but not necessarily so when SQLITE_MUTEX_FAST is used.
The mutex implementation does not need to make a distinction
between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
not want to.  SQLite will only request a recursive mutex in
cases where it really needs one.  If a faster non-recursive mutex
implementation is available on the host platform, the mutex subsystem
might return such a mutex in response to SQLITE_MUTEX_FAST.</p>

<p>The other allowed parameters to sqlite3_mutex_alloc() (anything other
than SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) each return
a pointer to a static preexisting mutex.  Six static mutexes are
used by the current version of SQLite.  Future versions of SQLite
may add additional static mutexes.  Static mutexes are for internal
use by SQLite only.  Applications that use SQLite mutexes should
use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
SQLITE_MUTEX_RECURSIVE.</p>

<p>Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
returns a different mutex on every call.  But for the static
mutex types, the same mutex is returned on every call that has
the same type number.</p>

<p>The sqlite3_mutex_free() routine deallocates a previously
allocated dynamic mutex.  SQLite is careful to deallocate every
dynamic mutex that it allocates.  The dynamic mutexes must not be in
use when they are deallocated.  Attempting to deallocate a static
mutex results in undefined behavior.  SQLite never deallocates
a static mutex.</p>

<p>The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
to enter a mutex.  If another thread is already within the mutex,
sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
SQLITE_BUSY.  The sqlite3_mutex_try() interface returns <a href="../c3ref/c_abort.html">SQLITE_OK</a>
upon successful entry.  Mutexes created using
SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread.
In such cases the,
mutex must be exited an equal number of times before another thread
can enter.  If the same thread tries to enter any other
kind of mutex more than once, the behavior is undefined.
SQLite will never exhibit
such behavior in its own use of mutexes.</p>

<p>Some systems (for example, Windows 95) do not support the operation
implemented by sqlite3_mutex_try().  On those systems, sqlite3_mutex_try()
will always return SQLITE_BUSY.  The SQLite core only ever uses
sqlite3_mutex_try() as an optimization so this is acceptable behavior.</p>

<p>The sqlite3_mutex_leave() routine exits a mutex that was
previously entered by the same thread.   The behavior
is undefined if the mutex is not currently entered by the
calling thread or is not currently allocated.  SQLite will
never do either.</p>

<p>If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), or
sqlite3_mutex_leave() is a NULL pointer, then all three routines
behave as no-ops.</p>

<p>See also: <a href="../c3ref/mutex_held.html">sqlite3_mutex_held()</a> and <a href="../c3ref/mutex_held.html">sqlite3_mutex_notheld()</a>.
</p><p>See also lists of
  <a href="objlist.html">Objects</a>,
  <a href="constlist.html">Constants</a>, and
  <a href="funclist.html">Functions</a>.</p>
